Formatting some characters in a cell to Bold causes new characters added to also be Bold

Hi All. I created a macro to insert some text, format the text upto the ":" in BOLD, and the rest back to Normal text.
It appears to work fine until I type on the end of the text which keeps defaulting back to to BOLD again??
Any ideas?

With ActiveCell:
    .FormulaR1C1 = "=""GSM " & Date & " Actions for "" &[[Allocated Worker]]&"": There was a bear who lived in a tree"""
    .Value = .Value
    
    With .Characters(Start:=1, Length:=InStr(1, .Value, ":", vbTextCompare)).Font

        .FontStyle = "Bold"

    End With
    With .Characters(InStr(1, .Value, ":", vbTextCompare), 2000).Font
 
        .FontStyle = "Regular"

    End With
End With